Method of stress-relieving free-sintered polytetrafluoroethylene



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Fl! n--g--u ATTORNEY FIGA United States Patent 3,076,233 METHOD 0F STRESS-RELIEVING FREE-SINTERED POLYTETRAFLUOROETHYLENE John S. Taylor, Wilmington, Del., assignor to "Ihe Budd Company, a corporation of Pennsylvania Filed Feb. 28, 1961, Ser. No. 92,207 3 Claims. (Cl. 18-55) This invention relates to a method of stress-relieving free-Sintered polytetrafluoroethylene (P.T.F.E. or Teiion) and related materials having similar behavior and has for an object the provision of improvements in this art.

There are some types of P.T.F.E. material which can be free-Sintered in large sizes without objectionable porosity or imperfections. One such material is known as Teflon 7 which is a very finely divided powder-like material prepared for such use. Sintered billets of this material are used for forming thin sheets by skiving them therefrom. First a billet is preformed in a mold, say in the range of 10 to 12 inches long by about 10 inches outside diameter and with a core hole of about 4 inches diameter, under a high pressure in the range of 2000 p.s.i. Such a billet may weigh 50-75 lbs. It is removed from the mold and Sintered free of confinement or restraint in an oven.

It is found that after sintering the billet has changed shape considerably and that tape which is skived therefrom has residual stresses which produce objectionable results in the use of the tape. If the tape is treated to relieve these stresses and thereby stabilize it, as by sintering, the dimensions of the tape are changed.

Some improvement can be achieved by slow-cooling the Sintered billet but even after long cooling periods there are still very considerable locked-in stresses. Besides, the slow-cooling or soaking procedure is wasteful of equipment and time.

According to the present invention these diculties are overcome in a very simple Way.

When the billet has cooled down to a fairly low temperature, 250 F. or below (or reheated if cooled), it is placed in a mold and compressed against the direction of expansion during sintering. Usually the expansion is endwise of the axis, i.e., along the line of original compression in forming the billet, with corresponding contraction in the transverse direction. The mold for recompression may be the same one which was used in forming the billet, with a core or mandrel small enough to be inserted in the hole in the billet, and the same range of pressure is used, in the example about 2000 p.s.i. If a temperature lower than about 250 F. is used during recompression, a greater pressure is required.

This recompression forces the Sintered billet back into approximately the same size or smaller than the original unsintered billet. The recompressed Sintered billet expands when relieved of compression and nally reaches a stabilized size, say after 24 hours or longer.

IIf tape is now skived from the recompressed billet and resintered in use, as after wrapping on a conduct-or or other core, it is found that there is practically no change in the dimensions of the tape. This shows that the stresses which have been `locked in during cooling after sintering have been satisfactorily relieved by recompression.

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`In order to give a visual understanding of the invention illustration is provided wherein:

FIG. 1 shows a body of unsintered material in a mold ready to be compressed to form a billet;

FIG. 2 shows a billet after it has been preformed;

FIG. 3 shows a billet after being free-Sintered in an oven;

FIG. 4 shows a sintered billet being recompressed;

FIG. 5 shows the iinal stress-relieved billet; and

FIG. 6 shows a sheet being skived from a billet.

The billet B, of suitable P.T.F.E. or similar acting material, is formed (FIG. 2) in a mold M under pressure of a ram R at say 2,000 p.s.i. The billet, for example, will have a diameter (O.D.) of /16" with a length of V27/16"; and to receive a mandrel to assist in turning for skiving has a core hole of 4" diameter (I.D.).

FIG. 3 shows the billet B after being free-Sintered. It will now, for example, be 13% long, 91%6 O.D., and 3% LD.

FIG. 4 shows the Sintered billet B after being recompressed in a mold M by a ram R at a pressure of say 2,000 p.s.i. at 250 F. (greater pressure if cooler). The mold may be the same as that used for forming the billet with a core mandrel to lit the core hole. The recompressed billet size may be, for example, 105/16" O.D., 35/3 I D. and 11% long.

After recompression and standing, say for 24 hours, the size of billet B"' may be, for example, 101A" O D., 3% I.D. and 121/32" long.

'It is seen that the billet increases in length and decreases in outer and inner diameter during free sintering and that it is shortened and thickened in its wall during recompression.

In FlGS. 3 and 5 the next prior shapes are shown in broken lines.

FIG. 6 shows the billet B being skived by a knife K to form a sheet S, say 3 mils thick.

Tape and other products made according to the present method give every indication that stress relief has been quite fully achieved even in large billet sizes.

While one embodiment of the inventoin has been described for purposes of illustration it is to be understood that there may be various embodiments and modifications within the scope of the invention.

What is claimed is:

l. The method of stress-relieving a free-Sintered body of iluorocarbon material normally having locked-in stresses after cooling, which comprises, recompressing it at a temperature under approximately 250 F. after sintering in a direction against the direction of expansion during sintering with provision for limited expansion transversely of the line of compression.

2. The method of stress-relieving a free-Sintered billet of fluorocarbon material which has been compressed axial-l ly in a mold prior to sintering and which increases in length during sintering and cooling, which comprises, placing the free-Sintered billet in a laterally oversized mold and compressing it at a temperature under approximately 250 F. axially with the sides retained by the mold during the nal stage of compression.

3. The method of making a stress-relieved Sintered billet of polytetrafluoroethylene, which comprises, compressing powdered polytetratluoroethylene axially in a mold under a pressure of around 2,000 p.s.i., removing the billet from the mold and free-sintering it with increase in length and decrease in transverse cross section, placing the free-sintered billet in the same outside size mold, and recompressing it at a temperature under approximately 250 F. under a pressure of about 2,000 p.s.i. to reduce its length While conned on the sides during the nal stage of compression in the mold.

References Cited in the le of this patent UNITED STATES PATENTS 2,456,262 Fields Dec. 14, 1948 4 Haroldson et al June 7, 1961.

OTHER REFERENCES 

1. THE METHOD OF STRESS-RELIEVING A FREE-SINTERED BODY OF FLUOROCARBON MATERIAL NORMALLY HAVING LOCKED-IN-STRESSES 